Cannot rotate a camera on a motion path that is targeted to an empty.
Based on the description of @batFINGER, I was able to perform the steps to allow for camera rotation during the animation.

Setup:


empty for tracking (target)
empty for camera (dolly)
camera
BezierCircle (path)


Constrains:


for the camera empty:


follow path "BezierCircle" (don't forget to click "animate path")
Track to "empty for tracking" (make sure to set -Z and Y)

for the camera itself


constraint: child of "empty for camera"



This should now have the camera tracking the "empty for tracking"


go to camera view and select your camera: it is still possible to rotate the camera